This invention relates to a method and apparatus for the detection and monitoring of beta and alpha particles, and in particular to the use of a single deep layer silicon detector in combination with a microcomputer to monitor air quality by measuring and discriminating among the energy levels of air-borne radioactive particles.
Air pollution due to plutonium dust is extremely harmful to the human body. However, slight deviations above the legally permissable concentrations are difficult to detect because of the presence of daughter nuclides of natural radon and thoron isotopes. Plutonium emits alpha particles with an energy level of 5.3 meV while the daughter nuclides emit alpha particles with energy levels of 6.0 meV and 7.68 meV, but instruments using this basis for differentiation must allow for the fact that concentrations of radon and thoron daughters vary widely depending upon atmospheric as well as ground conditions.
The problem is complicated when there is need as well to detect the presence of beta emitting fission products. In a standard proportional type ionization counter, alpha particles may lose their energy and generate an electrical response from the detector as beta particles. The `straggled` alpha pulses become virtually indistinguishable from the beta pulses.
Proportional counters which operate with two energy channels are common. The higher energy alpha particles will be recorded at one voltage, while both alpha and the lower energy particles will be recorded at another voltage. Except for alpha straggling, the difference between the two will be a measure of beta activity.
Similarly, in a plutonium background compensating monitor, one channel will measure the higher energy alpha particles emitted by radon daughters, and another channel will measure the lower energy of alpha particles emitted by plutonium. An attempt is made to eliminate false measurement due to straggling by substracting a fraction of the higher energy pulse from the plutonium count.
Multichannel analyzers which record the true energy spectrum of both alpha and beta particles are often complicated and expensive. Differentiation of background and straggling counts requires the imposition on data of average counting rates arrived at experimentally, or curve fitting based on assumptions of normal distribution. By design, these instruments often neglect the need for instantaneous response in favor of accuracy by cumulative measurement, or, conversely, sound alarms repeatedly without reporting or analyzing minor deviations from a cumulative pattern.
No single instrument of the prior art efficiently combines the functions of measuring beta and alpha activity, as well as distinguishing beta and alpha activity from ambient background noise.
It is an object of this invention to provide an improved air monitor to record both alpha and beta activities and the energy of each pulse.
It is another object of this invention to provide an improved air monitor which rapidly distinguishes energy peaks due to alpha activity from natural radon and thoron daughters.
It is a further object of this invention to provide an improved air monitor to record natural beta activity present and discriminate in favor of beta emitting contaminants.
It is another object of this invention to provide an improved air monitor which operates remotely and automatically, recording both instantaneous and longterm radiation activity.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.